US6754101B2 - Refresh techniques for memory data retention - Google Patents
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- US6754101B2 US6754101B2 US10/423,350 US42335003A US6754101B2 US 6754101 B2 US6754101 B2 US 6754101B2 US 42335003 A US42335003 A US 42335003A US 6754101 B2 US6754101 B2 US 6754101B2
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C16/00—Erasable programmable read-only memories
- G11C16/02—Erasable programmable read-only memories electrically programmable
- G11C16/04—Erasable programmable read-only memories electrically programmable using variable threshold transistors, e.g. FAMOS
- G11C16/0408—Erasable programmable read-only memories electrically programmable using variable threshold transistors, e.g. FAMOS comprising cells containing floating gate transistors
- G11C16/0433—Erasable programmable read-only memories electrically programmable using variable threshold transistors, e.g. FAMOS comprising cells containing floating gate transistors comprising cells containing a single floating gate transistor and one or more separate select transistors
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C2216/00—Indexing scheme relating to G11C16/00 and subgroups, for features not directly covered by these groups
- G11C2216/02—Structural aspects of erasable programmable read-only memories
- G11C2216/10—Floating gate memory cells with a single polysilicon layer
Definitions
- This invention relates to memory arrays, and more particularly relates to data retention in such memory arrays.
- Oxide leakage Oxide leakage occurs even when the power is off and is a function of the amount of charge stored on the floating gate as well as the physical oxide characteristics (such as thickness and structure quality). Oxide leakage is unavoidable and is best addressed by fabrication process techniques.
- One apparatus embodiment of the invention is useful in a digital memory system including a memory cell arranged to store charge corresponding to first logical value when the quantity of stored charge lies within a first range of values and corresponding to a second logical value when the quantity of stored charges lies within a second range of values.
- the integrity of the data represented by the charge may be tested by apparatus comprising a bit line coupled to the cell and arranged to conduct charge on the bit line in response the charge stored in the cell and a charge integrity estimating module.
- the module is operative during a first mode of operation to detect whether the quantity of the charge lies within the first range of values or the second range of values, and is operative during a second mode of operation to detect whether the quantity of the charge lies within a third range of values comprising a subset of the first range of values.
- the module also is operative during a third mode of operation to detect whether the quantity of the charge lies within a fourth range of values comprising a subset of the second range of values.
- One method embodiment of the invention is useful in a digital memory system including a memory cell arranged to store charge corresponding to first logical value when the quantity of stored charge lies within a first range of values and corresponding to a second logical value when the quantity of stored charges lies within a second range of values and also including a bit line coupled to the cell.
- the integrity of the data represented by the charge may be tested by a method comprising detecting during a first mode of operation whether the quantity of the charge lies within the first range of values or the second range of values and detecting during a second mode of operation whether the quantity of the charge lies within a third range of values comprising a subset of the first range of values.
- the method comprises detecting during a third mode of operation whether the quantity of the charge lies within a fourth range of values comprising a subset of the second range of values.
- a data can be retained in a memory array with a degree of accuracy previously unattained.
- FIG. 1 illustrates the arrangement of FIGS. 1A and 1B.
- FIGS. 1A and 1B are schematic diagrams of one embodiment of the invention.
- FIG. 2 is a schematic diagram of the memory cell shown in FIG. 1 A.
- FIG. 3 is a schematic diagram of the PMOS transistors shown in FIG. 2 .
- FIG. 4 is a schematic diagram of the NMOS transistors shown in FIG. 2 .
- FIG. 5 is a voltage waveform diagram illustrating one embodiment of operation during the verify 0 mode of operation.
- FIG. 6 is a voltage waveform diagram illustrating one embodiment of operation during the verify 1 mode of operation.
- FIG. 1 illustrates a digital memory system 30 including a charge integrity estimating module 35 , a bit line 50 and a memory cell 52 that is arranged to store charge corresponding to first logical value when the quantity of stored charge lies within a first range of values and corresponding to a second logical value when the quantity of stored charges lies within a second range of values.
- the first range of values may correspond to a logical 0 and the second range of values may correspond to a logical 1.
- Module 35 includes a single-ended sense amplifier that has a built-in reference voltage generator 40 with noise suppression capability.
- Single-ended bit line 50 is sampled and held by a transmission gate 60 at the same time the bit line is floated or precharged (in anticipation of the integration time when charge is conducted on the floating bit line in response to charge stored in the cell). If there is noise on the bit line, the sampled reference voltage includes this noise, and it thus becomes common mode (i.e., ignored by differential sense amplifier 70 ).
- the sampled voltage stored in transmission gate 60 is then increased for a pull-up sense amplifier (or decreased for a pull-down sense amplifier) by a fixed fraction of the supply voltage by a capacitive divider circuit, such as transistor capacitors 42 and 44 , to form a reference voltage. This reference voltage is used by differential pull-down type sense amplifier 70 .
- transistors 72 - 75 form a cross-coupled pair of transistors for differential sense amplifier 70 .
- a control circuit 80 of module 35 includes access transistors 82 and 84 that couple bit line 50 and voltage generator 40 to the internal nodes of sense amplifier 70 .
- transistors 82 and 84 turn off, isolating the internal nodes of amplifier 70 .
- a transistor 86 determines when amplifier 70 is energized by coupling to a supply voltage VDD.
- a transistor 88 precharges bit line 50 to a supply voltage VDD.
- Bit line 50 is coupled to the q_bit node of sense amplifier 70 by transistor 82 .
- Bit line 50 also is coupled to transmission gate 60 over a path 90 .
- Control circuit 80 also includes inverters 92 - 96 .
- precharge transistor 88 is turned off, (when the preL signal becomes VDD)
- transmission gate 60 also is turned off, thereby isolating bit line 50 from voltage generator 40 .
- the source/drain nodes of transistor 42 are pulled low, coupling the Vref node down by a fixed fraction of the supply voltage VDD in order to generate the reference voltage for amplifier 70 on node Vref.
- the fraction is determined by the relative capacitance of transistor 42 and the node capacitance at node Vref, which includes the capacitance of transistor 44 .
- the node Vref capacitance can be adjusted by changing the size of transistor 44 , which is connected to a fixed voltage, such as ground potential or a reference potential VSS.
- the voltage on bit line 50 is transferred to node q_bit in amplifier 70 .
- the voltage on bit line 50 is generated in response to charge stored in cell 52 .
- the time during which charge is flowing is known as the integration period.
- the senpL signal is pulled low.
- transistors 82 and 84 are turned off after one inverter delay resulting from inverter 92 .
- sense amplifier 70 is energized when transistor 86 is turned on.
- sense amplifier 70 After being energized, sense amplifier 70 compares the values at q_ref and q_bit to determine the amount of charge stored in memory cell 52 , which determines the logical value stored in cell 52 .
- the value of q_bit is adjusted by the interaction of cell 52 with bit line 50 .
- module 35 also includes a transistor stack 110 comprising transistors 114 and 116 that are coupled to transistor pair 72 - 73 as shown, and a transistor stack 120 comprising transistor 124 and 126 that are coupled to transistor pair 74 - 75 as shown through conductors 123 and 127 - 128 .
- Stacks 110 and 120 place a load on sense amplifier 70 when activated. This arrangement cancels the offset of the sense amplifier or introduces a margin in a given direction.
- amplifier 70 generates an output voltage derived from the charge stored in the memory cell and the reference voltage on a path 102 after being transmitted through an inverter 104 .
- a reference voltage output is transmitted through an inverter 106 to an output path 108 .
- the reference voltage output inverter 106 is used to make the internal sense amplifier loads symmetric. Without inverter 106 , the sense amplifier would have a characteristic offset due to load asymmetry.
- the embodiment shown in FIG. 1 addresses any disturbance event that is specifically caused during power-on state of the memory array.
- an exemplary form of memory cell 52 comprises a non-volatile memory cell that is coupled to a bit line 50 and a word line 214 .
- the cell includes a node 216 that comprises a conductive material, such as metal or polysilicon, as well as various sources of reference voltages 220 - 224 .
- an NMOS field effect transistor 230 comprises a gate 232 , a source 233 , a drain 234 , a substrate 235 and an electrically insulating oxide layer 236 that is capacitively coupled between node 216 and a voltage source 222 .
- the source and drain are n type material and the substrate is p type material.
- the source and drain are separated by a channel region 237 .
- Electrodes 238 and 239 connect the source and drain in a current path 241 that extends from bit line 50 to voltage source 222 .
- Substrate 235 is connected to a voltage source 223 .
- Source 233 is connected to a source of voltage 222 .
- Transistor 250 is constructed like transistor 230 .
- Transistor 250 includes a gate 252 that is connected to word line 214 , a drain 254 that is connected to bit line 50 and a source 253 that is connected to drain 234 .
- Source 253 and drain 254 are connected in path 241 .
- a PMOS transistor 260 includes a gate 262 connected to node 216 , a source 263 , a drain 264 , a substrate 265 and an electrically insulating oxide layer 266 that is capacitively coupled between node 216 and voltage sources 220 - 221 .
- the source and drain are p type material and the substrate is n type material.
- the source and drain are separated by a channel region 267 .
- Electrodes 268 and 269 connect the source and drain in a current path 271 .
- Voltage source 220 is connected to the source and drain, and voltage source 221 is connected to substrate 265 .
- An NMOS field effect transistor 270 is constructed like transistor 260 .
- Transistor 270 includes a gate 272 that is connected to node 216 , and includes a source 273 , and a drain 274 that are connected to voltage source 222 .
- a channel region 277 separates the source and drain.
- An electrically insulating tunneling oxide layer 276 is located under gate 272 and is capacitively coupled between node 216 and voltage source 222 .
- the area of layer 276 is smaller than the area of layer 266 . Typically, the area of layer 276 is 5% or less of the area of layer 266 .
- the surface area of transistor 270 i.e., the surface area of the source, drain and channel
- the surface area of transistor 260 i.e., the surface area of the source, drain and channel
- the surface area of transistor 270 is 5% or less of the surface area of transistor 260 .
- the sources, drains, channel areas and substrates of each of transistors 230 , 250 , 260 and 270 are fabricated from semiconductor materials.
- planar structures are used to couple to the charge storage node 216 .
- field assisted tunneling is used to modulate the charge stored on node 216 .
- L the length of the transistor
- W the width of the transistor
- m the multiple of the transistor
- node 216 is a floating charge storage node whose potential is varied by modulating the charge stored on node 216 .
- Transistor 260 is a large PMOS device that is used to modify the potential at node 216 by capacitive coupling.
- a conventional controller 290 determines the voltage of sources 220 - 224 during various modes of operation.
- an erase operation that removes charge from node 216
- electrons are extracted from node 216 by raising terminal 222 to a high voltage.
- Transistor 270 is small relative to transistor 260 , and transistor 270 therefore forms a small portion of the capacitance at node 216 (e.g., the capacitance of node 216 supplied by transistor 270 is much smaller than the capacitance of node 216 supplied by transistor 260 ). Therefore, most of the applied source 222 voltage is dropped across the oxide layer 276 of transistor 270 .
- Electron extraction from node 216 is achieved when the high positive potential at source 222 attracts electrons from node 216 and causes them to tunnel through oxide layer 276 .
- controller 290 typically causes the voltages of sources 220 - 224 to be switched to the following values: Vp, source 220 , is approximately 0 volts; Vnwell, source 221 , is tied to Vp, source 220 ; Ve, source 222 , is approximately 4 volts; and VSS, nodes 223 - 224 , are approximately 0 volts.
- Electrons are injected into floating node 216 during a programming operation by raising terminals 220 - 221 to a high voltage. Terminals 220 - 221 can be tied together in a typical cell. Since transistor 260 forms the majority of the capacitance at node 216 , only a small amount of the applied source 220 - 221 voltage is dropped across the oxide layer 266 of transistor 260 . In other words, the percentage of source 220 - 221 voltage dropped across layer 266 is much lower than the percentage of source 222 voltage dropped across layer 276 . As a result, due to the operation of transistor 260 , the potential of node 216 rises significantly when high voltage is applied from sources 220 - 221 .
- Cells like cell 52 typically are organized in an array of cells.
- the voltage sources like source 222 are biased to half the source 222 program voltage so as not to cause any disturbance.
- controller 290 typically causes the voltages of sources 220 - 224 to be switched to the following values: Vp, source 220 , is approximately 4 volts; Vnwell, source 221 , is tied to Vp, source 220 ; Ve, source 222 , is approximately 0 volts; and VSS, nodes 223 - 224 are approximately 0 volts. If no program is desired for the cell, then Ve, source 222 , is approximately 2 volts.
- transistors 230 and 250 are used to read data from cell 52 . Erase and programming operations require large voltages in the range of 4 volts, and they result in threshold shift and device performance degradation over time.
- the read operation is accomplished by raising the voltage of word line 214 , and the voltages of sources 220 - 221 to typical chip voltages, such as approximately 1.2 volts in 0.13 micrometer technology. If the potential of node 216 is high enough (e.g., when cell 52 is erased), transistor 230 is switched on. If transistor 250 also is switched on, a current flow in path 241 causes the potential of bit line 50 to decrease. If the potential of node 216 is low (e.g., cell 52 is programmed and net negative charge is present on node 216 ), transistor 230 is not switched on, and no current flows in path 241 so that the voltage on bit line 50 remains high.
- controller 290 forms part of charge integrity estimating module 35 .
- the connections of controller 290 to the control signal paths of FIG. 1 have not been shown to avoid unnecessarily complicating FIG. 1 .
- Memory cell 52 has two verify modes: “verify 0” and “verify 1”. “Verify 0” reads a stored word with the sense margin for a “0” (little or no negative charge stored on node 216 ) decreased by a given amount (designed into the sense amplifier 70 ). “Verify 1” reads a stored word with the sense margin for a “1” (a range of substantial negative charge stored on node 216 ) decreased by a given amount.
- the shifts in sense margins are achieved by electrically skewing the sense amplifier 70 cross coupled pair 72 - 75 with stacks 110 and 120 . In another embodiment, stacks 110 and 120 are not used. Instead, the shifts in sense margins are achieved by varying the integration time during the first, second and third modes of operation.
- a bit to be verified is read in the normal read mode and the stored data is detected to be a 0 or a 1. If the data disturbance is so severe that there are failures even during the normal read mode, the corruption is unrecoverable. However, the refresh circuits used with most memory arrays continually scan memory cell 52 and fix disturbances so that data corruption is avoided. After the data is read and noted, if the bit is a 0, the bit is verified with the “verify 0” mode. Another read operation is issued to memory cell 52 , this time with the “verify 0” mode asserted. The stored bit is now read with a reduced sense margin for 0's. If there are any failures on the expected 0's in the bit, the location is erased more to enforce the weakly stored data.
- a “verify 1” cycle is similarly applied to the memory this time with the “verify 1” mode asserted. Any failures during the “verify 1” operation are re-enforced by re-programming the failing bit at the given location. Erase-verify for 0's and program-verify for 1's are repeated until there are no more failures in the normal read and verify read modes for all bits. The effects of power-on disturbances on the non-volatile storage cell are thus eliminated effecting an indefinite retention time as long as the power is on.
- module 35 allows single word program and erase operations (with the input data determining which bits in the word are to be programmed or erased). During a single-word erase operation, only the bits of the accessed word that correspond to input data 0 are erased. During a single-word program operation, only the bits of the accessed word that correspond to input data 1 are programmed.
- Module 35 is continually testing the memory incorporating cell 52 and is verifying each word of the memory. If a word fails a verify operation, that word is erased to fix any failing 0's and is programmed to fix any failing 1's. During the refresh operations, access to the memory is blocked. If this blocking is not desirable, various caching techniques can be used to minimize direct access to the non-volatile memory.
- transistor stacks 110 and 120 are added in parallel with the cross coupled sensing nmos pair 72 - 75 . These stacks can be turned on with the ver 0 H signal (verify 0 mode) and ver 1 H signal (verify 1 mode).
- ver 0 H signal ver 0 mode
- ver 1 H signal verify 1 mode
- sense amplifier 70 is skewed due to the physical size difference in the active cross coupled pair.
- verify 1 signal when the verify 1 signal is asserted, the sense amplifier is skewed in the other direction making the margin requirement for sensing a 1 smaller.
- the end result can be thought of moving the reference voltage (Vref) higher or lower during the verify operations. Instead of a fixed reference value, there is a voltage reference “band” within which no cells can reside (otherwise these cells will be “flagged” during verify operations).
- stacks 110 and 120 are not used. Instead, the integration time is varied during the first, second and third modes of operation.
- the verify 0 mode of operation is entered if during the first mode of operation, a read cycle determines that cell 52 stores a 0 (little or no negative charge).
- the integration time during the read cycle is shown as the time period between times T1 and T3.
- bit line 50 is precharged to VDD, and VREF is generated as previously explained.
- controller 290 causes word line 214 to go high, thereby turning on transistor 252 (FIG. 2 ). Controller also supplies ground potential to supply 222 at time T1.
- bit line 50 is compared with VREF by sense amplifier 70 , and the negative relationship can be detected by detecting module 130 at both times T2 and T3.
- bit line voltage is compared with VREF at time T2 during the verify 0 mode of operation. At time T2, the bit line voltage is greater than VREF, thereby indicating that node 216 needs to be erased.
- the integration time of bit line 50 is reduced from time T3 to time T2.
- the verify 1 mode of operation is entered if during the first mode of operation, a read cycle determines that cell 52 stores a 1 (a substantial amount of negative charge).
- the integration time during the read cycle is shown as the time period between times T1 and T3.
- bit line 50 is precharged to VDD, and VREF is generated as previously explained.
- controller 290 causes word line 214 to go high, thereby turning on transistor 252 (FIG. 2 ). Controller 290 also supplies ground potential to supply 222 at time T1.
- bit line 50 remains substantially at value VDD at both times T3 and T4 as shown by exemplary decay line 304 .
- the bit line voltage is compared with VREF by sense amplifier 70 , and the positive relationship can be detected by detecting module 130 at both times T3 and T4.
- bit line voltage is compared with VREF at time T4.
- the bit line voltage is less than VREF, thereby indicating that node 216 needs to be programmed.
- the integration time of bit line 50 is increased from time T3 to time T4.
- controller 290 and detecting module 130 are performed by controller 290 and detecting module 130 , and both may be implemented in hardware, software, or a combination of hardware and software.
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US10/423,350 US6754101B2 (en) | 2002-05-21 | 2003-04-25 | Refresh techniques for memory data retention |
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US10/151,981 US6816412B2 (en) | 2002-05-21 | 2002-05-21 | Non-volatile memory cell techniques |
US44501703P | 2003-02-04 | 2003-02-04 | |
US10/423,350 US6754101B2 (en) | 2002-05-21 | 2003-04-25 | Refresh techniques for memory data retention |
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